The hydrolysis of esters is limited by thermodynamic equilibrium, meaning that conversion of reactants to products is controlled by the ratio of the reactants, namely, the desired carboxylic acid alkyl ester, and water and reaction temperature. Conversion can be increased to some extent by increasing reaction temperature and/or by removing either of the reaction products, namely, the carboxylic acid or alcohol from the product mixture.
The hydrolysis of carboxylic acid methyl esters is a commercially significant reaction and is typically catalyzed by acid cation exchange resins. For example, U.S. Pat. No. 2,936,321 discloses a continuous process wherein an ester of a lower alkanoic acid such as methyl acetate is hydrolyzed in the presence of water and a catalyst consisting of a cation-exchange resin in its hydrogen form to produce a mixture of an acid (acetic acid), an alkanol (methanol), unreacted ester (methyl acetate), and water. The reaction mixture is withdrawn from the hydrolysis reactor and the ester is separated from the reaction mixture.
U.S. Pat. No. 5,502,248 discloses a continuous process for hydrolyzing carboxylic acid alkyl esters which utilizes a solid bed which acts as a catalyst for hydrolysis and as an adsorbent for at least one of the reaction products. The process is operated in simulated moving bed mode. A process embodiment is disclosed wherein the reactants are placed in contact with a strongly acidic macroreticular polymeric resin which functions as a hydrolysis catalyst and at least one solid which acts as an adsorbent for at least one hydrolysis product. Suitable adsorbents include molecular sieve carbon and activated carbon.
U.S. Pat. No. 4,016,180 discloses a low cost, two-stage adsorption-desorption process of concentrating dilute supplies of chemicals. The process is particularly adapted toward concentrating waste condensates derived from pulp-making operations such as the Kraft or sulfite processes, but may be applied toward treating all types of dilute organic or inorganic absorbable chemicals. The process comprises adsorbing a chemical fraction of the mixture to be concentrated onto activated carbon followed by regenerating the adsorbed chemicals and concentrating the same by fractional distillation whereupon the partially concentrated chemicals are again adsorbed, regenerated, subjected to a second fractional distillation concentration step, and recovered.
Example 1 of U.S. Pat. No. 4,016,180 discloses a process for concentrating the components of a waste liquor comprising methanol, furfural, acetic acid, sulfur dioxide and water. During the process, the initially adsorbed acetic acid is first substantially converted to methyl acetate during regeneration operations. Methyl acetate is not hydrolyzed to acetic acid when adsorbed onto the activated carbon adsorbent according to the Specification (col. 11, lines 60-68 and col. 12 lines 1-3) which states that "During the early stages of fractionation of course, the effluent leaving through line 112b can be recycled through line 148 or 149 in order to assure complete removal of the acetic acid from the system. It will be appreciated from the foregoing that in the operation of this embodiment there is no conversion of acetic acid to methyl acetate with consequent reconversion of the latter and accordingly the operation of the FIG. 2 is somewhat simpler."
Izumi and coworkers (Catalysis Today 33 (1997) 371-409) present a review of processes for hydrolyzing alkyl esters in the presence of solid acid catalysts. Only a few solid acid catalysts provide acceptable activity and stability for liquid phase hydrolysis of esters. Suitable solid acid catalysts include cation exchange resins such as Amberlyst 15, zeolites and cesium salts of 12-tungstophosphoric acid. These solid acid catalysts were tested for the hydrolysis of ethyl acetate at 60.degree. C. using a mol ratio of ethyl acetate to water of 38.8 to 1.
Researchers are searching for carboxylic acid alkyl ester hydrolysis catalysts which can be operated at higher temperatures than conventional hydrolysis catalysts in order to maximize conversion to the desired products of the corresponding carboxylic acid and alcohol while minimizing formation of undesirable byproducts such as dialkyl ethers which are difficult to separate from the product mixture.